In an image forming apparatus of electrophotography, including a copying machine, a printer, etc., an electrostatic latent image according to image data of an original document is formed on a photoconductor rotating in a sub-scanning direction. This is achieved by exposing the surface of the photoconductor uniformly charged in advance with an exposure device (an optical writing device), and by causing toner to be adhered to the latent image on the photoconductor with a developing device, a toner image is formed on the photoconductor.
The toner image is then transferred onto a record sheet (a transfer sheet) with a transfer device. Thereafter, the toner image is fixed to the record sheet by heating and pressing the toner image on the record sheet with a fixing device, including for example a fixing roller including a built-in fixing heater.
In such an image forming apparatus, generally, the surface temperature of the fixing device (e.g., the fixing temperature of the fixing device) is controlled to accord with a target temperature. The surface temperature of the fixing device is detected with a sensor. According to a detected temperature, the electrifying duty ratio (the ratio of an electrifying or non-electrifying time per a unit time) relative to the fixing heater of the fixing device is controlled so that the fixing temperature of the fixing device accords with the target temperature. As a method of controlling the electrifying duty ratio relative to a fixing heater of a fixing device, it is known to control the electrifying duty ratio based on a temperature difference between a detected fixing temperature and a target temperature or based on both of the temperature difference and past control amounts.
In a known image forming apparatus performing control of the fixing temperature of a fixing device, a fixing device is provided with a fixing heater having a relatively large heating value to avoid forming an inferior image when the voltage of the commercial power source has been lowered to a lower limit or to complete warming up of the fixing device within a period of time when the power has been turned on or the apparatus has returned to the standby state from a power saving mode.
In such an image forming apparatus in which a fixing heater having a relatively large heating value is provided in a fixing device and in which the fixing temperature of the fixing device is controlled by way of controlling the electrifying duty ratio relative to the fixing heater, however, controlling the electrifying duty ratio is performed under similar conditions regardless of variation in the heating value among fixing heaters or variation in the voltage of commercial power sources at premises of users. Therefore, for example, if an AC input voltage of a commercial power source at a premise of a user exceeds a rated voltage, the fixing heater unnecessarily consumes a large amount of power.
To avoid such consumption of an unnecessarily large amount of power, in image forming apparatuses described in JP laid-opened publications No. 2000-29348 and No. 11-161098, a voltage of a commercial power source is detected. The electrifying duty ratio relative to a fixing heater of a fixing device is then restricted with a detected voltage of the commercial power source so that the maximum power consumption of the fixing heater is equal to or smaller than a value.
In such an image forming apparatus in which a voltage of a commercial power source is detected for controlling the fixing temperature of a fixing device, however, immediately after the power of the apparatus has been turned on or immediately after the apparatus has returned to the standby state from a power saving mode, the fixing device including a built-in fixing heater in a low temperature state is rapidly heated to be in a high temperature state that a satisfactory image can be formed and the electrical resistance value of the fixing heater rapidly changes with rising of temperature. Thus, an input voltage of the commercial power source cannot be correctly detected. Accordingly, the above-described controlling of the electrifying duty ratio relative to the fixing heater such that the maximum power consumption of the fixing heater to be equal to or below a value cannot be performed and countermeasures against an abnormal voltage cannot be taken.
Therefore, it is necessary to start controlling of the electrifying duty ratio relative to the fixing heater after the electrical resistance value of the fixing heater has been stabilized. This, however, increases the amount of time to heat the fixing device to be in a state that a satisfactory image can be formed immediately after the power has been turned on or immediately after the apparatus has returned to the standby state from a power saving mode.